Well, Galadriel and Turgon were first cousins. That makes Idril and Celebrian second cousins, and Eärendil and Arwen/Elladan/Elrohir third cousins. And then Elrond is their third cousin, once removed. Some of the phrases might be wrong, but I think I've got it right.

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Ah, I can see it that way too now.
The answer I was looking for was 'fourth cousins'. Elrond was a fourth cousin to his own children as follows:
1st cousins: Luthien and Earwen
2nd cousins: Dior and Galadriel
3rd cousins: Elwing and Celebrian
4th cousins: Elrond and Elladan/Elrohir/Arwen

Following the line as you did shows that Elrond and his grandson were also four th cousins!

Do you suppose that all the inbreeding is what caused their ears to be pointed?